Air conditioning systems are routinely employed within automobiles and other vehicles for creating comfortable conditions within the passenger compartment for the vehicle occupants. At outside temperatures above about 70.degree. F., it is difficult to maintain a comfortable passenger compartment temperature without first cooling the air that is being blown into the passenger compartment.
Typically, cooling of the air is accomplished by first compressing an appropriate refrigerant, such as the commonly used fluorocarbons (known as freon) or other alternative refrigerants. Within an automobile, the engine-driven compressor compresses the vaporized refrigerant, thereby significantly raising the temperature of the refrigerant. The refrigerant then flows into a condenser where it is cooled and returned to its liquid state; thus, the heat added to the refrigerant in the compressor is transferred out of the system. The cooled liquid refrigerant is then sprayed through an expansion valve into an evaporator where it is again vaporized. The heat of vaporization required for vaporizing the refrigerant is drawn from the incoming outside air, which is blown around the evaporator. Any excess humidity contained within the incoming air is removed as condensation on the evaporator, thereby also drying the incoming air. The cooled, dry air then enters the passenger compartment of the vehicle.
The materials and components within the air conditioning system must be capable of withstanding extremely demanding conditions, particularly, the materials used to form the components within the engine driven compressor. The compressor contains many mating components which continuously wear against each other during operation of the air conditioning system, while also being subject to significant pressures due to the compressed refrigerant. Appropriate lubricants are provided throughout the compressor at these bearing surfaces, so as to prevent excessive wear and galling between the mating materials. Typically in the past, a lubricant which is soluble in the refrigerant has been added directly in with the refrigerant when charging the compressor with the pressurized refrigerant prior to use. Since the conventional lubricants have been soluble within the refrigerant, the lubricant therefore moves freely through the compressor with the refrigerant, thereby providing lubrication where it is needed most between mating components.
However, due to environmental concerns, the current fluorocarbon-based refrigerants are being eliminated from use. Alternative refrigerants which alleviate environmental damage have been tested, with a 1,1,1,2-Tetrafluoroethane refrigerant, known as R134A, being a likely substitute. Unfortunately, conventional lubricants which have been previously (and successfully) employed with the fluorocarbon-based refrigerants are not soluble within the R134A refrigerant. Therefore the lubricant does not freely move throughout the compressor components when the new refrigerant is used and does not lubricate mating surfaces, as was the situation when the fluorocarbon-based refrigerants were used. The result is that during operation of the air conditioning system with the new R134A refrigerant, the bearing surfaces of the mating components are not lubricated and correspondingly they experience significantly higher incidence of wear.
Therefore, in the absence of an appropriate lubricant, it is necessary to provide a wear resistant material which is essentially self-lubricating. The desired material must be capable of not only providing sufficient lubricity, but must also be sufficiently strong to resist wear and galling during operation of the compressor. In addition, there are certain applications wherein the material must also be sufficiently ductile to permit the formation of a component from the material such as by swaging or other forming techniques. Therefore, the requirements of this material are many.
More particularly, in a five cylinder compressor which is in use within several automotive air conditioning systems, known generally as a "Wobble Plate" compressor, there is a "wobble plate" which has five pocketed regions. As shown in FIG. 1, the "wobble plate" has five sockets and is accordingly also referred to as a socket plate. As shown in FIG. 2 and 3, five high strength steel connecting rods are engaged within each of the five sockets, so as to reciprocate within each of the sockets. The socket plate material at each socket is then swaged around the ball end of the connecting rod.
Typically the socket plate has been formed from an aluminum-silicon alloy such as A356 or A357, and has performed quite satisfactorily with the previous refrigerant/lubricant combinations. However, in the absence of a suitable lubricant for use with the new refrigerant, excessive wear and even galling has occurred between the connecting rods and the aluminum-silicon sockets, whereby the softer socket material repeatedly attaches and welds itself to the harder steel connecting rod during use. This is not surprising since the conventional aluminum-silicon alloys are known for their good wear resistance, but only when lubrication is present since their hard surfaces can be quite damaging when lubrication is not present.
Therefore what is needed is an aluminum alloy for use in this socket plate which is particularly tolerant even when poorly lubricated, and which resists galling and wear by providing a certain degree of self-lubricity. In addition, the aluminum alloy must be sufficiently ductile to permit swaging of the material, yet sufficiently strong to contain the high pressure refrigerant over the repeated thermal cycling experienced within a typical automotive environment.